Wireless communication systems often employ power amplifiers for increasing the power of a signal. In a wireless communication system, a power amplifier is usually the last amplifier in a transmission chain (the output stage). High gain, high linearity, stability, and a high level of power-added efficiency (i.e., the ratio of the difference between the output power and the input power to DC power) are characteristics of an ideal amplifier.
In general, a power amplifier operates at maximum power efficiency when the power amplifier transmits peak output power. However, power efficiency tends to worsen as output power decreases. Recently, the Doherty power amplifier architecture has been the focus of attention not only for base stations but also for mobile terminals because of the architecture's high power-added efficiency.
A Doherty power amplifier typically includes two or more amplifiers such as a carrier amplifier and a peaking amplifier. These amplifiers are connected in parallel with their outputs joined by an offset transmission line, which performs impedance transformation. The peaking amplifier delivers current as the carrier amplifier saturates, thereby reducing the impedance seen at the output of the carrier amplifier. Thus, the carrier amplifier delivers more current to the load while the carrier amplifier is saturated because of a “load-pulling” effect. Since the carrier amplifier remains close to saturation, a Doherty power amplifier is able to transmit peak output power so that the total efficiency of the system remains relatively high.
The high efficiency of the Doherty architecture makes the architecture desirable for current and next-generation wireless systems. However, the architecture presents challenges in terms of semiconductor package design. Current Doherty power amplifier semiconductor package design calls for the use of discrete devices and integrated circuits that may involve one device that includes the carrier amplifier and a separate device that includes the peaking amplifier. These discrete devices are maintained a distance apart in the package in order to limit problems with crosstalk that can occur between the carrier and peaking amplifiers.
One source of crosstalk in the semiconductor package architecture is between arrays of signal wires, referred to as wire bond arrays, that may be connected between the various electrical devices making up each of the carrier and peaking amplifiers. That is, the performance of a Doherty power amplifier can be adversely affected by coupling (i.e., the transfer of energy from one circuit component to another through a shared magnetic or electric field) between adjacent wire bond arrays of the corresponding components of the Doherty power amplifier. Coupling can be of two types, electric (commonly referred to as capacitive coupling) and magnetic (used synonymously with inductive coupling). Inductive or magnetic coupling occurs when a varying magnetic field exists between current carrying parallel conductors that are in close proximity to one another, thus inducing a voltage across the receiving conductor.
Unfortunately, maintaining spatial distance between amplifiers in the package limits the potential for miniaturization of the semiconductor package. Limiting miniaturization is undesirable where low cost, a low weight, and a small volume are important package attributes for various applications.